1. Field of the Invention
The present invention relates to antenna apparatuses and communication terminal instruments for use in RFID systems or near field wireless communication systems, which communicate with counterpart devices through electromagnetic signals.
2. Description of the Related Art
In an RFID system or a near field wireless communication system, the use of which is growing in recent years, a communication antenna is mounted on each device for communications between portable electronic instruments such as portable phones or between a portable electronic instrument and a reader/writer. Of these antennas, International Publication Pamphlet No. WO 2009/142114 discloses antennas to be mounted on portable electronic instruments.
FIG. 1A is a cross-sectional diagram of a principle portion of a wireless IC device described in International Publication Pamphlet No. WO 2009/142114. FIG. 1B is a diagram illustrating a spatial arrangement of a power feed circuit board and a radiation board of the wireless IC device. The wireless IC device 1 includes a wireless IC chip 5 that processes predetermined radio signals, a power feed circuit board 4 that is connected to the wireless IC chip 5 and includes a power feed circuit including at least one coil pattern 23, and a radiation plate 3 that radiates transmission signals provided from the power feed circuit board 4, receives reception signals, and provides them to the power feed circuit board 4. The radiation plate 3 includes in part an opening portion 7 and a slit portion 6 connecting to the opening portion 7. The opening portion 7 of the radiation plate 3 overlaps an inner side region of the coil pattern 23 in plan view when viewed in a winding axis direction of the coil pattern 23, and areas of the inner side region and the opening portion 7 are nearly equal to each other.
In FIG. 1A, when a radio signal is being transmitted, a signal current flows from the wireless IC chip 5 to the coil pattern 23. An induced magnetic field H generated by that current distributes through the opening portion 7 as illustrated by dotted lines in the drawing. The induced magnetic field H causes induced currents I1 and I2 to flow in a surrounding portion of the opening portion 7, as illustrated in FIG. 1B. Here, the slit portion 6 is connected to the opening portion 7. Thus, the slit portion 6 limits the flows of these induced currents I1 and I2. As described above, in the radiation plate 3, the induced currents I1 and I2 are induced by the magnetic field H. This magnetic field causes the radio signal to be transmitted.
However, it is necessary to have the opening portion and the slit portion in the radiation plate. Thus, a restriction is imposed on the shape of the radiation plate, and a highly accurate positional relationship between the radiation plate and the coil conductor must be satisfied.